Recently, compared to a system which is necessary for plate making, such as a gravure printing system or flexographic printing system, inkjet recording apparatuses are widely used, because an image can be formed simply and at a lower cost.
In a field in which the image recording is conducted on goods or packaging by using an inkjet recording apparatus, there are many cases where materials having no ink absorptivity such as resin or metal are used. There is known an inkjet recording apparatus (for example, refer to Patent Document 1) which fixes ink on such a recording medium having no ink absorptivity, and has a recording head provided with nozzles for ejecting ink which is cured by irradiating light such as ultraviolet ray, and a light irradiating device provided with a light source for generating light for curing ink. In this apparatus, after ink ejected from the nozzle lands on the recording medium, the ink is cured by irradiating light onto this recording medium with the light irradiating device, and thus an image is formed.
Herein, if a time period from when the ink is impacted on the recording medium until when the light is irradiated is long, the dot diameter of the ink impacted on the recording medium is enlarged and bleeding or color mixture is caused, and the image quality drops. Accordingly, to reduce a time period from when the ink is impacted on the recording medium until when the light is irradiated, in a known inkjet recording apparatus, a light irradiating device is provided close to a recording head. Consequently, in order to prevent ink adhered to the surface with nozzles from being cured by light having been emitted from the light source and reached the nozzle formed surface of the recording head, the light irradiating device is provided with a cover member to cover the light source.
Further, in an inkjet recording apparatus which cures ink by using a light irradiating device, when illuminance of light emitted from a light source drops equal to or lower than a value by which ink can be well cured, due to the deterioration of the light source, ink ejected from the recording head and impacted on a recording medium is not well cured, and the image quality drops. Therefore, it is proposed that the illuminance of the light emitted from the power source be detected and when it drops equal to or lower than the reference value, the drop in the illuminance be reported to the user by a device which displays the drop on a display unit, or image recording be prohibited.
Hereupon, recently, cation curable ink has been proposed, which is an energy accumulation type that can be cured if it is irradiated for a long period of time even with a low illuminant light. In order to cure this cation curable type ink, a low electric power and low output ultraviolet light source such as a low pressure mercury lamp, cold cathode ray tube can be used.    [Patent Document 1] Tokkai No. 2003-145725
However, because, particularly in a low output type light source, the light emitting efficiency has a characteristic of changing with the temperature of a base part discharging electricity in the light source, or changing with the temperature difference between the light emitting tube and the base part. Accordingly, when the temperature inside a cover member rises due to heat generation of the light source and the temperatures of the base part and light emitting tube change, the light emitting efficiency of the light source changes. Accordingly, it is necessary to detect the illuminance of light emitted by the light source and confirm if the illuminance is high enough prior to starting image recording. After the light is turned on, the illuminance of light emitted by the light source rises for a certain time with a rise in the temperature of the light source, and then stabilizes. The time taken for stabilization of the illuminance is effected by the environmental temperature, the light time, and the time from when the light was turned off last time until the light was turned on this time. Accordingly, the time taken for stabilization of the illuminance depends on use environment of the apparatus in use, which causes a problem of detecting the illuminance before the illuminance stabilizes, or detecting the illuminance after a time longer than necessary has elapsed.
Further, when illuminance of light emitted from the light source is detected for evaluation and if the illuminance becomes equal to or lower than the reference value, this status is reported to the user, or image recording is prohibited. Then, the user is required to determine whether the drop in the illuminance is due to the life of the light source or due to drop in the light emitting efficiency, and take a necessary step, which causes a problem of placing a load on the user.
Accordingly, to solve a problem as described above, a first object of the invention is to provide an inkjet recording apparatus having a device that intermittently detects illuminance of a light source, calculates the illuminance-change-rate, and determines that the illuminace of the light source has stabilized, namely, has come to a peak, when the illuminance-change-rate has come into a predetermined range.
To solve another problem as described above, a second object of the invention is to provide an inkjet recording apparatus which maintains an optimum light emitting efficiency of a light source, and correctly determines the life of the light source so as to cure ink always well by emitting a proper amount of light, without placing a load on the user, thus providing high quality images.